This invention relates to retroreflectors and more particularly to oscillating retroreflectors for sweeping the plane of maximum reflected areal energy density through the plane of the source of radiative energy.
A retroreflector is a device which reflects incident radiative energy back toward its source. A well known retroreflector includes three planar reflecting facets mounted substantially at right angles to one another. Such a retroreflector is known as a corner cube reflector and has the property that any ray which is reflected off each of the three orthogonal surfaces will return to the source along a ray parallel to its path of incidence. It is important to note that an incident ray will be returned along a path parallel to the incident ray but not coincident with the ray. It is for this reason that the reflected energy directed toward the source is dispersed. That is, with a conventional corner cube retroreflector, the areal density of the reflected energy at the source is lower than the areal density of the incident beam upon the corner cube reflector. With a corner reflector having facets aligned precisely at .pi./2 radians, the areal density of the reflected radiation at the source is one-fourth the areal density at the plane of the retroreflector. This dispersion of the reflected energy often severely limits the usefulness of retroreflectors because of the difficulty in detecting the diffuse reflected energy at the location of the source.
One application for retroreflectors has been in the field of surveying. In such a case light from a source is reflected from a corner reflector back in the direction of the source, the reflected light is detected, and the travel time to the retroreflector and back to the source used as an indication of the distance between the source and the retroreflector. In some cases the light source is a laser. Because the photodetector used to detect the reflected light is governed by quantum effects, the photodetector may not respond at all if the areal density of the reflected radiation is too low. The range over which such surveying apparatus can be used, therefore, is limited because of this dispersion effect. Retroreflectors have also been used for precisely locating aircraft and ships. In such a case, the aircraft or ship has retroreflectors mounted on board which are adapted for reflecting electromagnetic radiation of a frequency used for radar interrogation. As before, with conventional retroreflectors the areal density of the reflected radiation is at best one-fourth the areal density of the radiation at the plane of the retroreflector. This dispersion effect may result in the return signals being undetectable. In general, therefore, there are significant numbers of applications of retroreflection in the processing of electromagnetic signals as well as in non-signal applications in which it would be advantageous to have retroreflected electromagnetic radiation arrive at its source with as great an areal density as possible.
It is an object of this invention to provide retroreflecting apparatus which increases the areal density of reflected radiation over that which can be achieved with the known corner cube retroreflectors. It is a further object to provide such apparatus which is inexpensive to fabricate and reliable in operation. Another object is to provide a reflector capable of returning electromagnetic radiation back to its source such that at some increment of time the intensity of the reflected radiation impinging on the source is substantially equal to that impinging upon the reflector. Still another object is to provide an array of such retroreflectors which may be mounted on the potential target of a high powered laser weapon, which array protects the target from damage and reflects the laser light impinging upon it back toward the laser weapon.